Free Radical Substitution Reactions of Alkanes/Halogenation of Alkanes

Contents

• Substitution reaction

• Free radicals

• Chain initiation

• Chain propagation

• Chain termination

• Uses of paraffins

Learning
Objectives

At the end of this lecture, student will be able to

• Explain the mechanism involved in free radical
substitution reactions of alkanes

Alkanes or Paraffins

• Alkanes are
also known as saturated hydrocarbons and sometimes as paraffins

• The name
“paraffin” is based on the Latin words parum + affinis =
“little affinity”.

• The
“little affinity” behind their Latin name referring to their
relatively low reactivity.

• They have
“little affinity” for other elements or compounds

Uses of alkanes

• Methane.
The largest amount of methane produced is used for heating purposes

• Ethane is
being used as heating fuel

• Propane
is an important raw material in petrochemistry

• Butane is
an important feedstock for the petrochemical industry

Properties of alkanes

• Saturated hydrocarbons

• Contains only carbon-carbon single bond

• C-H bond- non-polar covalent bond

• Alkanes- non-polar compounds

• Quite unreactive- contain only strong sigma bonds

• A compound containing a halogen atom covalently bonded to
an sp3 hybridized carbon atom. Given the symbol R-X

• Called as haloalkane or alkyl halide 

Preparation of haloalkanes

• Halogenation of alkanes

• Can also be prepared by addition of X₂ to
alkenes

• Halogenation of alkanes is common with Br₂ and
Cl₂

• Mixture of methane and chlorine gas- in dark and at room
temperature- no detectable change

• On heating or exposed to light- reaction occurs

• If allowed to react with more amount of chlorine

• Regioselectivity

• Treating propane with bromine gives a mixture consisting
of approximately 92% of 2-bromopropane and 8% of 1-bromopropane

Problem-01

•       Bromination

Free
Radical Substitution Reaction of Alkanes

• Chlorination of methane

Step 1

Mechanism of Halogenation of Alkanes

• Occurs as radical chain mechanism

• Involve three types of steps

1)      Chain
initiation

2)      Chain
propagation

3)      Chain
termination  

Mechanism of Free Radical Substitution Reaction

Step 1

•   Chain Initiation step

Step 2

• Chain propagation
step

Step 3

• Chain propagation
step

Step 4

• Chain termination
step

– coupling of 2 free radicals

• Chain propagation
step

– Formation of CH2Cl2

Summary

• Alkanes are also known as saturated hydrocarbons and
sometimes as paraffins

• The name “paraffin” is based on the Latin words
parum + affinis = “little affinity”.

• The “little affinity” behind their Latin name referring to
their relatively low reactivity.

• They have “little affinity” for other elements
or compounds

• Methane. The largest amount of methane produced is used
for heating purposes

• Ethane is being used as heating fuel

• Propane is an important raw material in petrochemistry

• Butane is an important feedstock for the petrochemical
industry

• Free radicals – highly reactive reaction intermediates
with odd electron formed by homolysis of covalent bond

• Free radicals – generated by heat or irradiation of light
or initiators

• Halogenation of alkanes takes place by free radical chain
reactions

• Free radical mechanism involves 3 steps – chain
initiation, propagation and termination

• Free radicals – generated in the initiation step

• One radical generates another radical with the formation
of product in the propagation step

• Chain termination takes place by coupling of 2 free
radicals

• Butane is an important feedstock for the petrochemical
industry